The present invention concerns thick-film overglaze inks and their use as a protective layer for electrical circuit structures, particularly multilayer designs and resistor networks on ceramic, preferably alumina, substrates.
The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuit structures is known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
U.S. Pat. No. 4,401,709 describes a thick-film overglaze ink comprising 60 to 90 weight % of a glass powder and 8 to 35 weight % of an organic vehicle. The glass powder of U.S. Pat. No. 4,401,709 is described as containing (a) 58 to 66 weight % of lead oxide, (b) 12 to 20% of a modifier consisting of (i) 2 to 6 weight % cadmium oxide, (ii) 4 to 8 weight % zinc oxide, (iii) 0.1 to 3 weight % barium oxide and (iv) 0.1 to 3 weight % antimony trioxide and (c) 20 to 27 weight % of a glass forming component consisting of (i) 0.1 to 2 weight % aluminum oxide, (ii) 14 to 20 weight % boron trioxide and (iii) 1 to 7 weight % silicon dioxide.
Overglaze inks are utilized to provide mechanical and environmental protection for the portion of films of various functions predominately resistor and conductor films which is not otherwise covered, e.g., by components.
It is generally recognized that overglaze films formed from inks such as herein described should possess: a thermal coefficient of expansion reasonably close to the substrate being utilized and, preferably, also reasonably close to that of the various functional films making up the circuit; good mechanical strength; minimal porosity in general and no through porosity; and most importantly, good chemical compatibility with the underlying conductor and resistor films.
Overglaze inks conventionally contain lead oxide to lower the firing temperature substantially below that of the functional films already on the board. The inherent disadvantage in a high lead content, however, is that increasing the lead content, however, is that increasing the the lead content of the ink also increases the probability of surface chemical reduction of the lead oxide when fired in an inert, e.g., nitrogen, atmosphere. Surface reduction of the lead oxide will interfere with sintering and reduce the flow of the ink. This latter problem is the more serious one since an ink fired in nitrogen must flow as it would if fired in air. The invention of the present application basically involves a low lead content (&lt;3 weight percent) overglaze composition.
It also must be appreciated that an overglaze ink must not react to a significant extent with any part of the circuit on the board. This is particularly true with resistors, since such reaction will normally have an adverse effect on the stability and resistance values of the resistors. To be acceptable, an overglaze film should not change the value of a resistor by more than five percent. A change of plus or minus two percent is the normal industry standard. Films formed from the novel overglaze inks of this invention readily fall within this standard acceptance criteria.
Although, in some instances, overglaze inks are fired at the same time as the underlying conductors and resistors, for reasons of economy, they are generally independently fired at peak temperatures substantially below the firing temperature of conductor and resistor films. Some conventional inks are fireable in only air or nitrogen.